It is desirable to detect a misfire condition during the operation of an internal combustion engine. One approach taken in the art to detect misfire involves assessing the degree of engine speed (RPM) variation. Although such an approach does not require additional circuit components, it does require relatively extensive software to perform the evaluation, and, further, is incapable of producing accurate and reliable results over the entire engine speed and load range on a higher number of cylinder engines.
Another approach involves the use of a so-called ion sense system. It is known that the combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. It is further known to apply a relatively high voltage across, for example, the electrodes of a spark plug just after the ignition operation to produce a current across the electrodes. Such current is known as an ion current. The ion current that flows is proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above, and is consequently indicative of the ionization throughout the entire cylinder as combustion occurs. The level or amount of ion current is indicative of the quality of the combustion event, or whether in fact combustion has occurred at all (e.g., a misfire condition).
Known ion current sensing systems generally include, in addition to an ignition coil, a capacitor or the like configured to store a voltage. The stored voltage is thereafter used as a "bias" voltage which is applied to the spark plug to generate the ion current. These systems therefore require additional circuitry for the storing and biasing operation, which may be undesirable in certain circumstances.
U.S. Pat. No. 5,814,994 to Hohner et al. discloses a system to generate an ion current and purports to provide a control circuit configured to drive an ignition transistor such that a saw-toothed collector current, or alternatively, a triangle-shaped current, results as a primary current. Although the circuitry of such a control circuit is not disclosed in detail, it is believed that such circuitry would be relatively complicated, and further, would require that the ignition transistor dissipate a relatively increased amount of power. These aspects may also be undesirable in certain circumstances.
Moreover, known ion sense systems, while capable of producing results over the entire engine speed and load range, even on a higher number of cylinder engines, generally require a microprocessor or the like, and thus are more expensive.
There is therefore a need to provide an apparatus for detecting a combustion condition, such as a misfire condition, that minimizes or eliminates one or more of the shortcomings as set forth above.